Modulation apparatus, demodulation apparatus, audio transmission system, program, and demodulation method

ABSTRACT

A problem is to increase possibility of extracting data from sound in a system which transmits data using the sound as a transmission medium. Frames are transmitted while shifting the transmission timing among a plurality of frequency bands, whereby tolerance for multipath fading, noise mixing, or the like is obtained and improvement of a substantial transmission rate can be expected compared to a case where one frame is transmitted using only one frequency band. When selecting blocks forming a frame, since a block is selected under a condition that a time period necessary for collecting sound, on which the blocks to be selected are superimposed, becomes shorter, even if a phenomenon, in which a substantial rate of data transmission decreases, occurs, it becomes possible to suppress a decrease in rate.

This application is a U. S. National Phase Application of PCTInternational Application PCT/JP2014/053266 filed on Feb. 13, 2014,which is based on and claims priority from JP 2013-032506 filed on Feb.21, 2013 and JP 2013-246685 filed on Nov. 28, 2013, the contents ofwhich is incorporated in its entirety by reference.

TECHNICAL FIELD

The present invention relates to a technique for transmitting data usingsound (sound wave) as a transmission medium.

BACKGROUND ART

As a technique for transmitting data using an audio signal or sound(sound wave) as a transmission medium, techniques described PatentLiteratures 1 and 2 are known. In the technique described in PatentLiterature 1, a modulation apparatus on a sound emission side modulatesa spread code with a data code, performs differential encoding for themodulated spread code, multiplies by a carrier signal, performs afrequency shift, and outputs the differential code as a modulatedsignal. A demodulation apparatus on a sound collection side appliesdelay detection to an input modulated signal with a delay time for onechip of the differential code, detects synchronization between thedelay-detected signal waveform and the spread code, and demodulates thedata code based on the peak polarity of the detected synchronizationpoint. In the technique described in Patent Literature 2, an electronicwatermark is embedded in an audio signal by means of amplitudemodulation, and the electronic watermark is extracted from the audiosignal based on temporal and intensity features of fluctuation inamplitude.

CITATION LIST Patent Literature

Patent Literature 1: JP-A-2010-288246

Patent Literature 2: JP-A-2006-251676

SUMMARY OF INVENTION Technical Problem

On the other hand, when transmitting data using sound, a system whichsuperimposes data on sound in a specific frequency band is known. Inthis system, the following problem is considered. For example, when theenvironment of data transmission is a space with much reverberant sound(reflected sound) or when a speaker as sound emission means and amicrophone as reception means do not directly face each other, theinfluence of reflected sound, so-called multipath fading, occurs, soundin a frequency band for data transmission is reduced or the volume leveldecreases, and superimposed data may not be extracted. Also, when noise(for example, brake sound of a vehicle, noise of equipment, or the like)at a frequency conforming to a frequency band for data transmission ismixed from the outside, superimposed data may not be extracted. In thesystem which superimposes data on sound belonging to a specificfrequency band, a phenomenon, such as multipath fading or noise mixing,occurs, and data may not be extracted from sound.

Accordingly, an object of the present invention is to increasepossibility of extracting data from sound in a system which transmitsdata using sound as a transmission medium.

Solution to Problem

In order to solve the problem as mentioned above, a modulation apparatusaccording to an aspect of the present invention includes: a delay unit,configured to delay a transmission start timing of a frame correspondingto one unit of transmission data by a predetermined time period; and amodulated signal generation unit, configured to generate a modulatedsignal by modulating a carrier wave in a frequency band which isdifferent depending on the transmission start timing by using the framewhose transmission start timing is delayed by the delay unit.

The modulation apparatus may be configured by further including a soundemission unit, configured to emit sound according to the modulatedsignal generated by the modulated signal generation unit.

Moreover, a demodulation apparatus according to an aspect of the presentinvention includes: a separation unit, configured to use a framecorresponding to one unit of transmission data and having a transmissionstart timing delayed by a predetermined time period to separate an audiosignal of sound emitted according to a modulated signal generated bymodulating a carrier wave in a frequency band which is differentdepending on the transmission start timing into signal componentsbelonging to the respective frequency bands; and a frame generationunit, configured to demodulate a block corresponding to a part of theframe for each predetermined time period based on each signal componentseparated by the separation unit, and connect blocks selected from ademodulated block group according to a prescribed selection method togenerate the frame.

It may be configured so that frequency bands of carrier waves modulatedby modulated signal generation unit are n frequency bands, where nindicates a positive integer, and the frame generation unit demodulatesthe block based on each signal component for each time period of 1/n ofa time period in which one frame is superimposed in any of the nfrequency bands.

It may be configured so that each of the frequency bands includes aplurality of narrow-band frequencies having bandwidth narrower than thecorresponding frequency band, the modulated signal generation unitgenerates the modulated signal by causing outputs of signals belongingto two narrow-band frequencies corresponding to the frame to be invertedbetween the signals according to a value of each bit of the frame, theseparation unit separates the audio signal into signals belonging to thetwo narrow-band frequencies included in each of the frequency bands, andthe frame generation unit compares a difference between the signalsbelonging to the two narrow-band frequencies with a threshold value anddecodes each value of the bits to demodulate the block.

It may be configured so that the frame generation unit calculates anupper envelope and a lower envelope of each signal component separatedby the separation unit and uses a time-varying value between thecalculated upper envelope and the lower envelope as the threshold value.

Moreover, an audio communication system according to an aspect of thepresent invention includes: a transmission apparatus for emitting anaudio signal, on which transmission data to be transmitted issuperimposed, as sound; and a reception apparatus for extracting thetransmission data from the sound emitted from the transmissionapparatus, wherein the transmission apparatus includes: a delay unit,configured to delay a transmission start timing of a frame correspondingto one unit of the transmission data by a predetermined time period; amodulated signal generation unit, configured to generate a modulatedsignal by modulating a carrier wave in a frequency band which isdifferent depending on the transmission start timing by using the framewhose transmission start timing is delayed by the delay unit; and asound emission unit, configured to emit the sound according to themodulated signal generated by the modulated signal generation unit, andthe reception apparatus includes: a sound collection unit, configured tocollect the sound emitted from the sound emission unit and output anaudio signal; a separation unit, configured to separate the audio signaloutput from the sound collection unit into signal components belongingto the respective frequency bands; and a frame generation unit,configured to demodulate a block corresponding to a part of the framefor each predetermined time period based on each signal componentseparated by the separation unit, and connect blocks selected from ademodulated block group according to a prescribed selection method togenerate the frame.

Moreover, a program according to an aspect of the present invention is aprogram which causes a computer to execute: a separation step of using aframe corresponding to one unit of transmission data and having atransmission start timing delayed by a predetermined time period toseparate an audio signal of sound emitted according to a modulatedsignal generated by modulating a carrier wave in a frequency band whichis different depending on the transmission start timing into signalcomponents belonging to the respective frequency bands; and a framegeneration step of demodulating a block corresponding to a part of theframe for each predetermined time period based on each signal componentseparated by the separation step, and connecting blocks selected from ademodulated block group according to a prescribed selection method togenerate the frame.

A demodulation method according to an aspect of the present inventionincludes: a separation step of using a frame corresponding to one unitof transmission data and having a transmission start timing delayed by apredetermined time period to separate an audio signal of sound emittedaccording to a modulated signal generated by modulating a carrier wavein a frequency band which is different depending on the transmissionstart timing into signal components belonging to the respectivefrequency bands; and a frame generation step of demodulating a blockcorresponding to a part of the frame for each predetermined time periodbased on each signal component separated by the separation step, andconnecting blocks selected from a demodulated block group according to aprescribed selection method to generate the frame.

Advantageous Effects of Invention

According to the present invention, it is possible to increasepossibility of extracting data from sound in a system which transmitsdata using sound as a transmission medium.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration example of an audiotransmission system according to an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration example of amodulation unit of a transmission apparatus.

In FIG. 3, (a) is a diagram showing an example of a frame structure ofdata, and (b) is a diagram conceptually showing the relationship betweena frame and blocks.

FIG. 4 is a conceptual diagram illustrating a transmission timing offrames.

FIG. 5 is a conceptual diagram illustrating a differential signal.

FIG. 6 is a block diagram showing a configuration example of ademodulation unit of a reception apparatus.

FIG. 7 is a flowchart showing a procedure example of envelopeprocessing.

FIG. 8 is a graph showing the result of envelope processing.

FIG. 9 is a block diagram showing a configuration example of a datadetection trigger generation unit.

FIG. 10 is a flowchart showing a procedure example of data detectionprocessing.

FIG. 11 is a conceptual diagram illustrating a detection procedure of asynchronization symbol.

FIG. 12 is a conceptual diagram illustrating a rule when a block isextracted to configure a frame.

FIG. 13 is a block diagram showing another configuration example of amodulation unit of a transmission apparatus.

FIG. 14 is a block diagram showing another configuration example of ademodulation unit of a reception apparatus.

FIG. 15 is a block diagram showing a configuration example of amodulation unit of a transmission apparatus or a demodulation unit of areception apparatus.

DESCRIPTION OF EMBODIMENTS

[1. Outline of Audio Transmission System]

An audio transmission system according to an embodiment of the presentinvention is a system which transmits and receives information to betransmitted with sound (sound wave) as a transmission medium. The audiotransmission system includes at least a transmission apparatus whichemits sound while superimposing information on an audio signal, and areception apparatus which collects sound and extracts information. Sincethe emission of sound with information superimposed thereon correspondsto the transmission of information, hereinafter, it is described asnecessary that the transmission apparatus transmits information.Furthermore, since the collection of sound with information superimposedthereon corresponds to the reception of information, hereinafter, it isdescribed as necessary that the reception apparatus receivesinformation.

Although the audio transmission system is used in, for example, thefollowing situations (1) to (3), the present invention is notnecessarily limited to these examples.

(1) Information for advertising goods or services is superimposed onsound and transmitted from a transmission apparatus which is provided ata location, such as a road or a store, at which a plurality of users arepresent, to a reception apparatus, such as a smartphone, which iscarried by each user.

(2) A television apparatus in a house functions as a transmissionapparatus, and information relating to, for example, a televisionprogram is superimposed on sound and transmitted from the televisionapparatus to a reception apparatus, such as a smartphone or a personalcomputer, which is used by a user.

(3) One of mobile devices, such as smartphones, which are carried bymultiple users functions as a transmission apparatus, the other mobiledevices function as a reception apparatus, and individual information,such as contact information of the user, is superimposed on sound andtransmitted from the transmission apparatus to the reception apparatus.

In this embodiment, information to be transmitted is repeatedlytransmitted in time series. For example, in the example of (1), the sameinformation for advertising goods or services is repeatedly transmittedfrom the transmission apparatus to the reception apparatus. Thereception apparatus performs processing, such as information display,when the information can be normally received. Of course, information tobe transmitted may be transmitted once, instead of being repeatedlytransmitted.

In the situations (1) to (3), information to be transmitted may besuperimposed on sound in an inaudible area or information superimposedon sound in an inaudible area may be further superimposed on music orvoice belonging to an audible area, such as background music.Alternatively, information to be transmitted may be superimposed onsound in an inaudible area while there is no music or voice in anaudible area.

[2. Overall Configuration of Audio Transmission System]

FIG. 1 is a block diagram showing a configuration example of an audiotransmission system. Here, although a minimum configuration including atransmission apparatus 1 and a reception apparatus 2 is shown forsimplification of description, each of the transmission apparatus 1 andthe reception apparatus 2 may include a configuration other than theconfiguration shown in the drawing.

The transmission apparatus 1 includes a modulation unit 10, an outputunit 11, and a speaker 12. The modulation unit 10 is an example of amodulation apparatus according to the present invention, and is meansfor modulating a carrier wave belonging to a high frequency band bytransmission data D to be transmitted and superimposing on audio data S.The term “high frequency band” used herein is a frequency band higherthan the upper limit value (from ten and several kHz to about 20 kHz) ofa frequency band of human audible sound. For example, in the example of(1), the audio data S is music, such as background music, or voice on aroad or at a store, and the transmission data D is information foradvertising goods or services. The audio data S and the transmissiondata D may be stored in, for example, a storage medium of thetransmission apparatus 1 or may be supplied from the outside of thetransmission apparatus 1 to the transmission apparatus 1. The outputunit 11 includes a D/A converter which converts a digital signal outputfrom the modulation unit 10 to an analog signal, and an amplifier whichamplifies the analog signal output from the D/A converter and suppliesthe amplified analog signal to the speaker 12. The speaker 12 is a soundemission unit for emitting sound according to the analog signal inputfrom the output unit 11. Emitted sound propagates through a space (theair) and is collected by a microphone 20 of the reception apparatus 2.

The reception apparatus 2 includes a microphone 20, an input unit 21,and a demodulation unit 22. The microphone 20 is a sound collection unitfor collecting sound emitted from the speaker 12 and outputting an audiosignal according to the sound. The input unit 21 includes an amplifierwhich amplifies the audio signal output from the microphone 20, and anA/D converter which converts the analog audio signal output from theamplifier to a digital signal. The demodulation unit 22 is an example ofa demodulation apparatus according to the present invention, anddemodulates the transmission data D from the digital signal output fromthe A/D converter. The transmission data D is a bit stream having “1”and “0”, and is used for a predetermined purpose, for example, issupplied to a display apparatus (not shown) connected to the receptionapparatus 2 and displayed as information on the display apparatus orsupplied to a communication apparatus (not shown) connected to thereception apparatus 2 and transmitted from the communication apparatusto the outside.

The configuration of each of the modulation unit 10 of the transmissionapparatus 1 and the demodulation unit 22 of the reception apparatus 2may be realized by hardware or may be realized by cooperation ofhardware and software. For example, as shown in FIG. 15, hardware of themodulation unit 10 when the configuration of the modulation unit 10 isrealized by cooperation of hardware and software is configured as acomputer. In this case, as shown in FIG. 15, the modulation unit 10includes at least a control unit 1000 that has a microprocessor, a RAM,and the like, and a storage unit 1001 which is a large capacity storage,such as a hard disk. The microprocessor of the control unit 1000 reads aprogram stored in the storage unit 1001 on the RAM and executes the readprogram, whereby each configuration (a delay device, an LPF, a VCO, anadder, or the like described below) of the modulation unit 10 isrealized. Also, hardware of the demodulation unit 22 when theconfiguration of the demodulation unit 22 is realized by cooperation ofhardware and software is the configuration shown in FIG. 15. In thiscase, the microprocessor of the control unit 1000 reads a program storedin the storage unit 1001 on the RAM and executes the read program,whereby each configuration (an HPF, a STFT unit, a subtracter, an LPF, aDC cut unit, a binarization unit, a data detection unit, a datadetection trigger generation unit, or the like described below) of thedemodulation unit 22 is realized. The modulation unit 10 and thedemodulation unit 22 may include a configuration (for example, anoperating unit, a display unit, a communication unit, or the like) otherthan the configuration illustrated in FIG. 15.

[3. Configuration of Modulation Unit in Transmission Apparatus]

FIG. 2 is a diagram showing a configuration example of the modulationunit 10 of the transmission apparatus 1. The modulation unit 10 includesan LPF 101 as a processing system for the audio data S, and includesLPFs 1021 to 1023, VCOs (Voltage-controlled oscillators) 1031 to 1033,delay devices 1041 and 1042, and an adder 105 as a processing system forthe transmission data D. The LPF 101 is connected to the adder 105. TheLPFs 1021 to 1023 are respectively connected to the adder 105 throughthe VCOs 1031 to 1033. The LPF 1021 and the LPF 1022 are connectedtogether through the delay device 1041, and the LPF 1022 and the LPF1023 are connected together through the delay device 1042. The detailsof the respective units will be described below.

[3-1. Structure of Transmission Data]

Prior to describing the specific processing contents of the respectiveunits of the transmission apparatus 1, first, the structure oftransmission data which is transmitted by the transmission apparatus 1will be described. As described above, transmission data to betransmitted is repeatedly transmitted from the transmission apparatus 1in time series. One unit of repeated transmission is called a frame.FIG. 3(a) is a diagram showing an example of the structure of a frame.One frame F has a synchronization symbol for finding the head of theframe, a header, in which information relating to the attribute of theframe, such as a frame length, is included, a payload, in which actualdata is included, and a footer corresponding to the rear end of theframe in order from the head of the frame. Each of the data length ofthe synchronization symbol and the data length of the header is apredetermined number of bits, for example, about several bits.

In the reception apparatus 2, one frame is divided into n (where n is apositive integer, and in this embodiment, for example, n=3) units, anddemodulated in the divided units. One divided unit is called a block.FIG. 3(b) is a diagram conceptually showing the relationship between aframe and blocks. One frame has three blocks a, b, and c having the samedata length. In the block a which is the head of the frame, thesynchronization symbol and the header are necessarily included. In theblock c which is the tail of the frame, the footer is necessarilyincluded. That is, each of the data length of the synchronization symboland the header and the data length of the footer is shorter than thedata length of one block.

[3-2. Frequency Band for use in Frame Transmission and TransmissionTiming]

The transmission apparatus 1 uses different frequency bands andrepeatedly transmits one frame in the respective frequency bands. Atthis time, the transmission apparatus 1 performs control to delay thetransmission start timing of each frame in each frequency band by apredetermined time period described below such that the transmissionstart timing is different between the frames. FIG. 4 is a conceptualdiagram illustrating the transmission start timing of a frame. In thisdrawing, the notation of “a” means the block a, the notation of “b”means the block b, and the notation of “c” means the block c. F1, F2,and F3 mean the frequency bands of carrier waves of transmission data.

As shown in FIG. 4, the transmission apparatus 1 starts the transmissionof each frame in each of the different frequency bands F1, F2, and F3while delaying by a predetermined time period, that is, a time periodcorresponding to 1/n (in this embodiment, 1/3) of a time periodnecessary for transmitting one frame (or a time period in which oneframe is superimposed in a certain frequency band). For example, thetransmission of the block a at the head of the frame starts at the timet1 in the frequency band F3, the transmission of the block a starts atthe time t2 in the frequency band F2, and the transmission of the blocka starts at the time t3 in the frequency band F1. Accordingly, thetransmission start timing of a frame next to the above-described frameis constantly delayed by a time period corresponding to 1/3 of thetransmission period of one frame such that the transmission start timingis set to the time t4 in the frequency band F3, the time t5 in thefrequency band F2, and the time t6 in the frequency band F1. Therefore,the above-described “1/3 of a time period necessary for transmitting oneframe” corresponds to the length of a time period necessary fortransmitting one block.

[3-3. Processing System for Transmission Data]

Returning to FIG. 2, the processing system for the transmission data Dof the modulation unit 10 will be described. LPFs 1021 to 1023 arefilters which remove a frequency component corresponding to a highfrequency band to limit the band of a baseband signal, and are calledNyquist filters. A Nyquist filter is generally configured by a FIRfilter which is called a cosine roll-off filter, and the order of afilter, a roll-off rate, or the like is determined according to anapplication condition. In the reception apparatus 2, since filtering byan LPF is performed for a received signal, each of the LPFs 1021 to 1023and LPFs 2241 to 2243 (see FIG. 6 described below) of the receptionapparatus 2 is configured by a root raised cosine roll-off filter suchthat a perfect Nyquist filter is implemented.

The transmission data D is filtered by the LPF 1021 and then input tothe VCO 1031. The VCOs 1031 to 1033 are transmitters whose frequencychanges depending on a control signal (here, a bit value of transmissiondata input to the VCO). The VCO 1031 outputs a signal in a frequencyband f3 to the adder 105 when the bit value of transmission data is 1,and outputs a signal in a frequency band f3′ to the adder 105 when thebit value of transmission data is 0. Accordingly, the frequency band f3and the frequency band f3′ are used as a pair. In this embodiment, thedifference between the values of signals belonging to two frequencybands in a pair is called a differential signal.

FIG. 5 is a conceptual diagram illustrating a differential signal. Asdescribed above, the signal in the frequency band f3 is output when thebit value of transmission data is 1, and the signal in the frequencyband f3′ is output when the bit value is 0. Accordingly, when the bitvalue of transmission data is 1, as shown in FIG. 5(a), the signal inthe frequency band f3 is output as a predetermined value (indicated by asolid line), and the signal in the frequency band f3′ is not output(indicated by a dotted line). On the other hand, when the bit value oftransmission data is 0, as shown in FIG. 5(b), the signal in thefrequency band f3 is not output (indicated by a dotted line), and thesignal in the frequency band f3′ is output as a predetermined value(indicated by a solid line). In this way, the signal in the frequencyband f3 and the signal in the frequency band f3′ have the magnituderelationship between the values of the signals being reversed, i.e.,inverted-relation outputs between them depending on the bit value.

It is determined that the bit value is 1 when the difference (f3−f3′)between the signal in f3 and the signal in f3′ exceeds a thresholdvalue, and it is determined that the bit value is 0 when the difference(f3−f3′) between the signal in f3 and the signal in f3′ is equal to orsmaller than the threshold value. Although a way of determining thethreshold value will be described in detail in the description of thereception apparatus 2, the threshold value dynamically changes accordingto the influence of multipath fading or the like, not a prescribed fixedvalue. The frequency band F1 described referring to FIG. 4 means a bandin which the frequency bands f1 and f1′ are combined, the frequency bandF2 means a band in which the frequency bands f2 and f2′ are combined,and the frequency band F3 means a band in which the frequency bands f3and f3′ are combined. That is, the frequency band F1 includes thefrequency bands f1 and f1′ having narrower bandwidth, the frequency bandF2 includes the frequency bands f2 and f2′ having narrower bandwidth,and the frequency band F3 includes the frequency bands f3 and f3′ havingnarrower bandwidth. In the present invention, since the frequency bandsf1, f1′, f2, f2′, f3, and f3′ have bandwidth narrower than the frequencybands F1, F2, and F3, these are called narrow-band frequencies. In thisembodiment, for example, near frequency bands are paired, such as thefrequency bands f1=18000 Hz, f1′=18400 Hz, f2=18800 Hz, f2′=19200 Hz,f3=19600 Hz, and f3′=20000 Hz. In the pairs, the time-varying waveformsof signal components belonging to the lower frequency bands f1, f2, andf3 are called normal signals, and the time-varying waveforms of signalcomponents belonging to the higher frequency bands f1′, f2′, and f3′ arecalled inverted signals.

Returning to FIG. 2, if transmission data (hereinafter, referred to asframe data) for one frame is input, each of the delay devices 1041 and1042 outputs transmission data while delaying by a time periodcorresponding to 1/3 of the transmission period of one frame, that is, atime period (hereinafter, referred to as 1/3 frame transmission period)necessary for transmitting one block. Accordingly, the delay device 1041outputs, to the LPF 1022, frame data which is delayed by the 1/3 frametransmission period from the timing, at which frame data is input to theLPF 1021. The VCO 1032 outputs a signal in the frequency band f2 to theadder 105 when the bit value of frame data output from the LPF 1022 is1, and outputs a signal in the frequency band f2′ to the adder 105 whenthe bit value of frame data is 0. Similarly, the delay device 1042outputs, to the LPF 1023, frame data which is delayed by the 1/3 frametransmission period from the timing, at which frame data is input to theLPF 1022. The VCO 1033 outputs a signal in the frequency band f1 to theadder 105 when the bit value of frame data output from the LPF 1023 is1, and outputs a signal in the frequency band f1′ to the adder 105 whenthe bit value of frame data is 0.

[3-4. Processing System for Audio Data]

Next, the processing system for the audio data S of the modulation unit10 will be described. The LPF 101 removes a frequency component in ahigh frequency band from the audio data S. The cutoff frequency of theLPF 101 is set to, for example, about the upper limit value (from tenand several kHz to about 20 kHz) of an audible frequency band such thatauditory quality of the audio data S by an audience can be secured and aband (modulation band) for use in modulation can be secured. The cutofffrequency becomes a lower limit frequency of the modulation band. Thisis because, if the cutoff frequency of the LPF 101 is too low, qualityat the time of the emission of the audio data S is deteriorated, and ifthe frequency of the modulation band is lowered in conformity with thelow cutoff frequency, sound at the time of the emission of a modulatedsignal belonging to the modulation band is recognizable to theaudience's ears. Conversely, if the cutoff frequency of the LPF 101 istoo high, it is not possible to widen the modulation band, and thetransmission rate of transmission data decreases. The signal output fromthe LPF 101 is input to the adder 105.

In the adder 105, a modulated signal based on the transmission data D isadded to an audio signal based on the audio data S. The audio signalwith the modulated signal added thereto is supplied to the output unit11, and sound based on the modulated signal and the audio signal isemitted from the speaker 12. Also, a case where the audio signal basedon the audio data S is not supplied to the adder 105 is considered. Inthis case, only the modulated signal is supplied to the output unit 11,and sound (audio signal) based on only the modulated signal is emittedfrom the speaker 12.

In the above-described configuration of the modulation unit 10, thedelay devices 1041 and 1042 function as a delay unit for delaying thetransmission start timing of a frame corresponding to one unit oftransmission data by a predetermined time period. The LPFs 1021 to 1023,the VCOs 1031 to 1033, and the adder 105 function as a modulated signalgeneration unit for generating a modulated signal by modulating acarrier wave in a frequency band which differs depending on thetransmission start timing by using the frame whose transmission starttiming is delayed.

[4. Configuration of Demodulation Unit in Reception Apparatus]

FIG. 6 is a block diagram showing a configuration example of thedemodulation unit 22 of the reception apparatus 2. The demodulation unit22 includes a bit decoding unit 220, a data detection unit 230, and adata detection trigger generation unit 240. An audio signal collected bythe microphone 20 and subjected to A/D conversion by the input unit 21is input to the bit decoding unit 220. In this case, since the audiosignal to be input includes an audio signal corresponding to thetransmission data D modulated by the transmission apparatus 1, the audiosignal which is input to the bit decoding unit 220 is called a modulatedaudio signal A. The bit decoding unit 220 converts the audio signalcorresponding to the transmission data D in the input modulated audiosignal A to binary data of “1” or “0” to decode a bit value, and outputsbinary data to the data detection unit 230. The data detection unit 230extracts the transmission data D from binary data output from the bitdecoding unit 220 at the timing at which a trigger signal is suppliedfrom the data detection trigger generation unit 240. Hereinafter, thedetails of the respective units will be described.

[4-1. Bit Decoding Unit]

The bit decoding unit 220 includes an HPF 221, a STFT unit 222,subtracters 2231 to 2233, DC cut units 2251 to 2253, and binarizationunits 2261 to 2263.

[4-1-1. HPF]

The HPF 221 removes a signal component in a low frequency bandcorresponding to the audio data S from the input modulated audio signalA and extracts a signal component in a high frequency band correspondingto the transmission data D. That is, the cutoff frequency of the HPF 221is set to the lower limit frequency of the modulation band.

[4-1-2. STFT Unit]

The STFT unit 222 is a separation unit for separating a signal outputfrom the HPF 221 into signal components belonging to the frequency bandsf1, f1′, f2, f2′, f3, and f3′ used at the time of the transmission.Specifically, the STFT unit 222 carries out short-time Fourier transform(STFT) for the signal output from the HPF 221, separates the signal intothe signal components belonging to the above-described frequency bandsf1, f1′, f2, f2′, f3, and f3′, and outputs the time-varying waveforms ofthe respective signal components. In the short-time Fourier transform atthis time, an overlap ratio is 50%, that is, the STFT unit 222 performsSTFT by half overlap. For example, the FFT length is 1024 samples, andone symbol sample length is 1536 samples, and a sampling frequency afterSTFT is 86.1328125 Hz. Although one symbol sample length is, forexample, 1, 1.5, 2 times or the like the FFT length, in this embodiment,one symbol sample length is 1.5 times the FFT length. The samplingfrequency after STFT is calculated from the FFT length and the overlapratio.

[4-1-3. Subtracter]

The subtracters 2231 to 2233 are provided corresponding to the pairs ofthe frequency bands f1, f1′, f2, f2′, f3, and f3′, and each calculatesthe difference between the normal signal and the inverted signal in thecorresponding frequency band. For example, the subtracter 2231 subtractsa signal value ch1′ of the inverted signal belonging to the frequencyband f1′ from a signal value ch1 of the normal signal belonging to thefrequency band f1, the subtracter 2232 subtracts a signal value ch2′ ofthe inverted signal belonging to the frequency band f2′ from a signalvalue ch2 of the normal signal belonging to the frequency band f2, andthe subtracter 2233 subtracts a signal value ch3′ of the inverted signalbelonging to the frequency band f3′ from a signal value ch3 of thenormal signal belonging to the frequency band f3. Accordingly, thedifferential signals ch1−ch1′, ch2−ch2′, and ch3−ch3′ corresponding tothe respective pairs of the frequency bands f1, f1′, f2, f2′, f3, andf3′ are obtained.

[4-1-4. LPF]

The LPFs 2241 to 2243 are provided corresponding to the respective pairsof the frequency bands f1, f1′, f2, f2′, f3, and f3′, and remove asignal component in a high frequency band from the differential signalsinput from the subtracters 2231 to 2233, and extract a signal componentin a frequency band to which a baseband signal belongs. As describedabove, the LPFs 1021 to 1023 of the transmission apparatus 1 and theLPFs 2241 to 2243 of the reception apparatus 2 are configured such thata perfect Nyquist filter is implemented.

[4-1-5. DC Cut Unit]

The DC cut units 2251 to 2253 are provided corresponding to therespective pairs of the frequency bands f1, f1′, f2, f2′, f3, and f3′,and extract baseband signals from signals output from the LPFs 2241 to2243. Specifically, the DC cut units 2251 to 2253 perform processing(envelope processing) for correcting an envelope for the signals outputfrom the LPFs 2241 to 2243, and remove a DC offset to extract thebaseband signals.

FIG. 7 is a flowchart showing a procedure of envelope processing. InFIG. 7, terms means as follows:

In: Input signal which is input from the LPF 2241 to the DC cut unit2251,

Out: Output signal which is output from the DC cut unit 2251,

Kp: P control coefficient (for example, 0.1) in the envelope processing,

Td: D control coefficient (for example, 1.0) in the envelope processing,and

Out′, Ed′: Values in previous processing (initial values are 0.0).

First, the DC cut unit 2251 obtains Ep, Hi Side, Low Side, Out (StepS10) according to the calculation formulas.Ep=In−Out′,upper envelope Hi Side of baseband signal=−abs(Ep−Ed′),lower envelope Low Side of baseband signal=abs(Ep−Ed′), andOut=Out′+Kp(Ep+Td×Ed)

Next, if In>Out on the upper envelope Hi Side of the baseband signal,and Out>In on the lower envelope Low Side of the baseband signal (StepS20; YES), the DC cut unit 2251 sets that Out=In and Ed=0 (Step S30). Ifthe above-described determination is negative (Step S20; NO), the DC cutunit 2251 sets that Ed=Ep (Step S40).

In this case, on the Hi Side, the DC cut unit 2251 sets an envelopefollowing the input signal In at the rising edge of the input signal Inand attenuates the envelope in a minus direction at the falling edge ofthe input signal In. This processing is performed, whereby followabilityto change in volume level of collected audio or burst noise is improved.On the Low Side, the DC cut unit 2251 performs reverse processing tothat described above, that is, sets an envelope following the inputsignal In at the falling edge of the input signal In and attenuates theenvelope in a plus direction at the rising edge. The DC cut units 2252and 2253 perform envelope processing using the input signals input fromthe LPFs 2242 and 2243 according to the same procedure as describedabove.

FIG. 8 is a graph showing an example of the relationship among awaveform Out (solid line) of a signal (baseband signal) output from theDC cut unit 2251, an upper envelope env_(p) (one-dot-chain line), alower envelope env_(m) (dotted line), and a threshold value th(two-dot-chain line) for use when binarization is performed based on adifferential signal. Although the threshold value th is a value betweenthe upper envelope env_(p) and the lower envelope env_(m), typically, anintermediate value between the upper envelope env_(p) and the lowerenvelope env_(m) is used. Accordingly, the threshold value th is a valuewhich temporally varies between the upper envelope env_(p) and the lowerenvelope env_(m) with temporal changes in the upper envelope upperenvelope env_(p) and the lower envelope env_(m).

[4-1-6. Binarization Unit]

As described above, each of the binarization units 2261 to 2263binarizes the baseband signal (here, the above-described differentialsignal) using the time-varying threshold value th, decodes the bitvalue, and outputs the bit value to the data detection unit 230.Specifically, each of the binarization units 2261 to 2263 outputs thebit value “1” when the signal value of the differential signal isgreater than the threshold value th at this time, and outputs the bitvalue “0” when the signal value of the differential signal is equal toor smaller than the threshold value th at this time. As described above,the threshold value th varies with temporal changes in the upperenvelope env_(p) and the lower envelope env_(m). For example, when thesignal reception strength of one of the frequency bands f1 and f1′ islowered due to the influence of multipath fading, noise mixing, or thelike, one of the upper envelope env_(p) and the lower envelope env_(m)fluctuates, and the difference between both envelopes decreases.Accordingly, when the threshold value is fixed to a prescribed value,since the difference between the upper envelope env_(p) and the lowerenvelope env_(m) decreases, and the differential signal leans toward theupper envelope or the lower envelope and becomes close to a flatwaveform, an error is likely to occur in bit determination. In contrast,in this embodiment, even when the difference between the upper envelopeenv_(p) and the lower envelope env_(m) decreases, and high precision isrequired for bit determination using the threshold value, since thethreshold value th is adjusted over time as the intermediate valuebetween the upper and lower envelopes, an error is unlikely to occur inbit determination. With this, tolerance for multipath fading or noisemixing is improved, and precision of bit determination increases.

[4-2. Data Detection Trigger Generation Unit]

As described above, although the transmission data D is superimposed onthe audio data S, for example, when temporarily or intermittentlytransmitting the transmission data D, the transmission data D is notsometimes superimposed on the audio data S. In this case, it isefficient that the data detection unit 230 performs data detection onlyin a time period during which the transmission data D is superimposed onthe audio data S. Accordingly, the data detection trigger generationunit 240 gives notification of the timing at which data detection startsto the data detection unit 230. FIG. 9 is a block diagram showing theconfiguration of the data detection trigger generation unit 240. Thedata detection trigger generation unit 240 includes FFT units 2411 to2413, normalization units 2421 to 2423, a multiplier 243, and a signallevel calculation unit 244.

The FFT units 2411 to 2413 are provided corresponding to the respectivepairs of the frequency bands f1, f1′, f2, f2′, f3, and f3′, carry outFFT (Fast Fourier Transform) for the differential signals ch1−ch1′,ch2−ch2′, and ch3−ch3′ input from the subtracters 2231 to 2233, andoutput the resultant spectrums. The overlap ratio in the FFT at thistime is, for example, one of 25%, 50%, 75%, or no overlap. Accordingly,if the FFT length is, for example, 512 samples and the overlap ratio is25%, FFT is performed at an interval of 128 samples.

Next, the normalization units 2421 to 2423 normalize the spectrumsoutput from the FFT units 2411 to 2413. The multiplier 243 calculatesthe product of the elements of the spectrums obtained from thenormalization units 2421 to 2423. With this, a so-called runningspectrum is obtained.

In a time period during which the transmission data D is superimposed onthe audio data S, the differential signals ch1−ch1′, ch2−ch2′, andch3−ch3′ input from the subtracters 2231 to 2233 correspond to thebaseband signals. For example, in the FFT with the FFT length of 512samples, while the maximum frequency is about 29.06 Hz (FFT: the order Nin the fast Fourier transform=43), a demodulated baseband signal becomesclose to a square wave and thus corresponds to a harmonic, and if thisis taken into consideration, it is experimentally understood that themaximum frequency is about 33.64 Hz (N≅50). In contrast, in a timeperiod during which the transmission data D is not superimposed on theaudio data S, since a signal input from each of the subtracters 2231 to2233 corresponds to noise, a spectrum is distributed in a widerfrequency band than a case where the transmission data D is superimposedon the audio data S.

Accordingly, the signal level calculation unit 244 can estimate a signallevel by calculating the ratio of frequency components equal to or lessthan N=50, which are assumed that a baseband signal close to a squarewave is included, in the entire spectrum. That is, as the ratio of thefrequency components equal to or less than N=50 in the entire spectrumis great, the transmission data D is highly likely to be superimposed onaudio, that is, the signal level is great. The signal level calculationunit 244 outputs a trigger signal to cause the data detection unit 230to start data detection when the estimated value exceeds a thresholdvalue. In this way, the signal level calculation unit 244 determineswhether or not the transmission data D is superimposed on the audio dataS by measuring the running spectrum of the differential signals of thefrequency bands f1, f1′, f2, f2′, f3, and f3′ before the LPFs 2241 to2243 are carried out, and performs data detection only when it isdetermined to be superimposed.

[4-3. Data Detection Unit]

The data detection unit 230 extracts transmission data from bit dataoutput from the binarization units 2261 to 2263. FIG. 10 is a flowchartshowing the operation of the data detection unit 230. In FIG. 10, first,the data detection unit 230 acquires bit data output from thebinarization units 2261 to 2263 (Step S21).

Next, the data detection unit 230 searches for a synchronization symbol(Step S22). In this step, for example, the data detection unit 230acquires a bit stream at every two bits with an initial bit in CH1 bitdata in the frequency band F1 as a start position (hereinafter, referredto as a search start bit) (see FIG. 11(a)). As described above, sincethe STFT unit 222 performs STFT by half overlap, and one symbol samplelength (1536 samples) is 1.5 times the FFT length (1024 samples), thebaseband signal is expanded three times. For this reason, the datadetection unit 230 acquires a bit stream at every two bits.

As described above, since the data length of the synchronization symbolis a predetermined number of bits, when a bit stream having apredetermined number of bits is acquired from the search start bit, thedata detection unit 230 performs determination about whether or not thebit stream coincides with the a prescribed bit stream of thesynchronization symbol. If the acquired bit stream coincides with thesynchronization symbol, the data detection unit 230 progresses tosubsequent processing. When the acquired bit stream does not coincidewith the synchronization symbol, the data detection unit 230 performsdetermination about whether or not a bit stream having a predeterminednumber of bits from the search start bit coincides with the bit streamof the synchronization symbol for CH2 bit data in the frequency band F2different from before (see FIG. 11(b)). Then, when no synchronizationsymbols are found in the CH1 bit data, the CH2 bit data, or the CH3 bitdata, the data detection unit 230 returns to the CH1 bit data in theinitial frequency band (F1), shifts the position of the search start bitby one bit from the last position, and performs determination aboutwhether or not a bit stream having a predetermined number of bits fromthe search start bit coincides with the bit stream of thesynchronization symbol to re-search a synchronization symbol (see FIG.11(c)). The data detection unit 230 repeats the processing until asynchronization symbol is found.

If a synchronization symbol is found, the data detection unit 230further acquires a bit stream having a predetermined number of bits atevery two bits from the position of a bit corresponding to the rear endof the synchronization symbol in bit data where the synchronizationsymbol is found. The bit stream corresponds to the header of the frame.Since the frame length is described in the header, the data detectionunit 230 performs decoding and error detection only for the header anddetects the frame length (Step S23).

Next, the data detection unit 230 divides the frame length by the numberof blocks (in this case, three) in one frame, thereby obtaining the datalength of one block. Then, the data detection unit 230 extracts theblocks a, b, and c from bit data output from the binarization units 2261to 2263 under the following condition and connects the blocks togenerate a frame (Step S24).

FIG. 12 is a conceptual diagram illustrating a condition when blocks areextracted to generate a frame. In FIG. 12, a1, b1, b2, c1, and c2 arethe same blocks as a, b, and c notated by the same alphabetical letters,but for ease of understanding of the description of blocks to beselected, the number 1 or 2 is attached for distinction. The intervalbetween the times t1 to t7 is a time period (1/3 of the time periodnecessary for transmitting one frame described above, and hereinafter,referred to as one block transmission period) necessary for receivingone block. It is assumed that the current time is t7, and blocksreceived by the reception apparatus 2 in the respective frequency bandsF1, F2, and F3 to the time t7 are stored in a storage unit (not shown)of the reception apparatus 2 (demodulation unit 22).

The data detection unit 230 selects the remaining blocks b and cnecessary for forming a frame according to a prescribed selection methodwith the block a received at the time t7, that is, a block (in thedrawing, the block a1) at the head of one frame as a start point. Theprescribed selection method includes the following four procedures. Thedata detection unit 230 attempts frame decoding and error detection inorder from the procedure 1 to the procedure 4, and when transmissiondata for one frame can be correctly demodulated in a certain procedure,does not perform any subsequent procedures.

Procedure 1: At the current time t7 at which the block a1 is received,the block b1 and the block c1 which are received in the frequency bandsF2 and F3 different from the block a1 are selected (the block a1, theblock b1, and the block c1 surrounded by a solid line in the drawing).That is, in the procedure 1, the blocks are respectively selected fromall frequency bands in the 11 block transmission periods. Accordingly, atime period necessary until the reception apparatus 2 collects sound, onwhich transmission data for one frame is superimposed, is a transmissionperiod for one block.

Procedure 2: The block b1 received in the frequency band F2 differentfrom the block a1 at the current time t7, at which the block a1 isreceived, and the block c2 received in the same frequency band F1 as theblock a1 at the time t6 one block before the current time t7, areselected. That is, in the procedure 2, the blocks are selected from aplurality of frequency bands in an arbitrary combination in a timeperiod longer than the transmission period for one block and shorterthan the time period necessary for transmitting one frame.

Procedure 3: The block b2 received in the same frequency band F1 as theblock a1 at the time t5 two blocks before the current time t7 and theblock c2 received in the same frequency band F3 as the block a1 at thetime t6 one block before the current time t7 are selected (the block a1,the block b2, and the block c2 surrounded by a dotted line in thedrawing). That is, in the procedure 3, the blocks are selected from onefrequency band in the time period necessary for transmitting one frame.

Procedure 4: The block b2 received in the same frequency band F1 as theblock a1 at the time t5 two blocks before the current time t7 and theblock c1 received in the frequency band F3 different from the block a1at the current time t7, at which the block a1 is received, are selected.That is, in the procedure 4, the blocks are respectively selected from aplurality of frequency bands in an arbitrary combination in the timeperiod necessary for transmitting one frame. The procedure 4 is usedwhen there is a large influence of multipath fading, noise mixing on anyfrequency band, or the like, compared to the procedure 3.

If all of the blocks a, b, and c are gathered at the current time t7after data detection starts, a substantial time necessary for datadetection in the procedure 1 is t7−t6 (that is, 1/3 of the time periodnecessary for transmitting one frame). Accordingly, in the procedure 1,when all blocks forming one frame are successfully detected, of theprocedures 1 to 4, a substantial transmission rate is the highest. Thatis, a time period necessary until the microphone 20 collects audio, onwhich these blocks are superimposed, is shortest.

Accordingly, when data detection is successfully performed in theprocedure 2, a substantial time necessary for data detection is t7−t5(that is, 2/3 of the time period necessary for transmitting one frame).Therefore, the procedure 2 has a second highest substantial transmissionrate after the procedure 1.

When data detection is successfully performed in the procedures 3 and 4,a substantial time necessary for data detection is t7−t4 (that is, thesame period as the time period necessary for transmitting one frame).Accordingly, when data detection is successfully performed in theprocedures 3 and 4, a substantial transmission rate is the lowest. Thatis, a time period necessary until the microphone 20 collects audio, onwhich these blocks are superimposed, is the longest. In the procedures 3and 4, while the influence of multipath fading, noise mixing, or thelike can be suppressed, the transmission rate does not change comparedto a case where a single frequency band is used.

Accordingly, if there is no multipath fading or the like which adverselyaffects transmission quality, in this embodiment, the time periodnecessary for transmitting one frame takes, at the shortest, a timeperiod of 1/3 of a time period necessary for transmitting one framewithout dividing into frequency bands. Even when the adverse influenceleads to degradation of transmission quality, in this embodiment, if atime period necessary for transmitting one frame without dividing intofrequency bands is taken at the longest, there is a high possibilitythat one frame can be transmitted.

When the blocks are selected in order from the procedure 1 to theprocedure 4 and frame decoding and error detection are attempted, insummary, this means that the reduction in the time period necessaryuntil the microphone 20 collects sound, on which blocks to be selectedare superimposed, is preferentially made. That is, the data detectionunit 230 selects blocks according to an algorithm in which a time periodnecessary for collecting sound, on which selected blocks aresuperimposed, becomes shorter.

The data detection unit 230 outputs a frame generated through decodingand error detection as transmission data (Step S25). When an erroroccurs during the above-described processing, the data detection unit230 returns the initial processing of Step S21 and attempts datadetection from the next bit again.

In the configuration of the demodulation unit 22 described above, theSTFT unit 222 functions as separation unit for separating an audiosignal output from the microphone 20 into signal components belonging tothe respective frequency bands. The subtracters 2231 to 2233, the DC cutunits 2251 to 2253, the binarization unit 2261, and the data detectionunit 230 function as a frame generation unit for demodulating a blockcorresponding to a part of a frame for each predetermined time periodbased on each signal component separated by the STFT unit 222 andconnecting blocks selected from a demodulated block group to generatethe frame. The frame generation unit selects blocks according to amethod selected according to a prescribed selection method, for example,a selection method in which the time period necessary until themicrophone 20 collects sound, on which the selected blocks aresuperimposed, becomes shorter. The data detection trigger generationunit 240 functions as a determination unit for performing determinationabout whether or not the transmission data D is superimposed on soundcollected by the microphone 20.

According to the above-described embodiment, the differential signalsusing different frequency bands are used, whereby the SN ratio isimproved compared to a case whether the differential signals are notused. The threshold value when the baseband signals are binarized basedon the differential signals is dynamically controlled according to thecollection situation of sound belonging to these frequency bands,thereby improving precision of bit determination. The frames aretransmitted while shifting the transmission timing among a plurality offrequency bands, whereby tolerance for multipath fading or noise mixingis obtained. While a frequency band which is influenced by a phenomenon,such as multipath fading or noise mixing, temporally fluctuates,according to the above-described embodiment, since the frames aretransmitted while shifting the transmission timing among a plurality offrequency bands, it becomes possible to increase options when selectingblocks forming a frame and to extract data from sound in a frequencyband which is less influenced by the above-described phenomenon. Sincethere are many blocks which are options when selecting blocks forming aframe compared to a case where one frame is transmitted using only onefrequency band, if blocks are selected according to a prescribedselection method, it becomes possible to reduce the time periodnecessary for collecting sound on which selected blocks aresuperimposed. Accordingly, improvement in a substantial transmissionrate can be expected compared to a case where one frame is transmittedusing only one frequency band, and for example, even if a phenomenon,such as multipath fading or noise mixing, in which a data transmissionrate decreases, occurs, it becomes possible to suppress the decrease inthe substantial transmission rate.

MODIFICATION EXAMPLES Modification Example 1 Configuration Example ofModulation Unit

The modulation unit 10 shown in FIG. 2 may have a configuration shown inFIG. 13. A modulation unit 10 a according to Modification Example 1includes, as a processing system for audio data S, an LPF 101 which isthe same as shown in FIG. 2, and includes, as processing system fortransmission data D, delay devices 1041 and 1042 which are the same asshown in FIG. 2, six transmitters 1061 to 1063 and 1061′ to 1063′, andvariable resistors 1071 to 1073, and an adder 108. That is, thismodification example is different from the above-described embodiment inthat the modulation unit 10 a includes the transmitters 1061 to 1063 and1061′ to 1063′, the variable resistors 1071 to 1073, and the adder 108.In the variable resistor 1071, one end is connected to the transmitter1061, the other end is connected to the transmitter 1061′, and a movableterminal which is an output terminal moving between the terminals atboth ends is connected to the adder 108. In the variable resistor 1072,one end is connected to the transmitter 1062, the other end is connectedto the transmitter 1062′, and a movable terminal which is an outputterminal moving between the terminals at both ends is connected to theadder 108. In the variable resistor 1073, one end is connected to thetransmitter 1063, the other end is connected to the transmitter 1063′,and a movable terminal which is an output terminal moving between theterminals at both ends is connected to the adder 108. In thismodification example, the transmitters 1061 to 1063 and 1061′ to 1063′,the variable resistors 1071 to 1073, and the adder 108 function as amodulated signal generation unit. Each configuration of the modulationunit 10 a may be realized by hardware or may be realized by cooperationof hardware and software.

The transmitter 1061 outputs a signal in a frequency band f3, and thetransmitter 1061′ outputs a signal in a frequency band f3′. Thetransmitter 1062 outputs a signal in a frequency band f2, and thetransmitter 1062′ outputs a signal in a frequency band f2′. Thetransmitter 1063 outputs a signal in a frequency band f1, and thetransmitter 1063′ outputs a signal in a frequency band f1′. Iftransmission data is input to the variable resistor 1071, when the bitvalue of the transmission data is “1”, the variable resistor 1071 movesthe movable terminal such that the resistance value decreases from thetransmitter 1061 to the adder 108 and the resistance value increasesfrom the transmitter 1061′ to the adder 108. With the movement of themovable terminal, the intensity of the signal in the frequency band f3output from the transmitter 1061 gradually increases, and the intensityof the signal in the frequency band f3′ output from the transmitter1061′ gradually decreases. When the bit value of the transmission datais “0”, the variable resistor 1071 moves the movable terminal such thatthe resistance value decreases from the transmitter 1061′ to the adder108 and the resistance value increases from the transmitter 1061 to theadder 108. With the movement of the movable terminal, the intensity ofthe signal in the frequency band f3′ output from the transmitter 1061′gradually increases, and the intensity of the signal in the frequencyband f3 output from the transmitter 1061 gradually decreases.

Similarly, the variable resistor 1072 moves the movable terminal suchthat the resistance value decreases from the transmitter 1062 to theadder 108 and the resistance value increases from the transmitter 1062′to the adder 108 when the bit value of the transmission data is “1”, andmoves the movable terminal such that the resistance value decreases fromthe transmitter 1062′ to the adder 108 and the resistance valueincreases from the transmitter 1062 to the adder 108 when the bit valueof the transmission data is “0”. The variable resistor 1073 moves themovable terminal such that the resistance value decreases from thetransmitter 1063 to the adder 108 and the resistance value increasesfrom the transmitter 1063′ to the adder 108 when the bit value of thetransmission data is “1”, and moves the movable terminal such that theresistance value decreases from the transmitter 1063′ to the adder 108and the resistance value increases from the transmitter 1063 to theadder 108 when the bit value of the transmission data is “0”.

In the embodiment, for example, when a differential signal is switchedfrom the frequency band f1 to the frequency band f1′, the signal in thefrequency band f1′ is instantaneously generated substantially at thesame time the signal in the frequency band f1 is instantaneouslyvanished. In contrast, in this modification example, when a differentialsignal is switched from the frequency band f1 to the frequency band f1′,the intensity of the signal in the frequency band f1 gradually decreasesand the intensity of the signal in the frequency band f1′ graduallyincreases over a time period longer than a time period necessary forinstantaneously switching from the frequency band f1 to the frequencyband f1′ in the embodiment. That is, when causing the outputs of signalcomponents belonging to these narrow-band frequencies to be invertedbetween them, the inversion is gradually performed over a comparativelylong period described above. As in the embodiment, for example, if adifferential signal is instantaneously switched from the frequency bandf1 to the frequency band f1′, since the spectrum of audio to be emittedrapidly changes, the audience may feel a sense of discomfort in auditorysensation. Accordingly, as in this modification example, if switching ismade gradually from the frequency band f1 to the frequency band f1′, itis possible to perform control such that the sense of discomfort isreduced. The above-described variable resistors 1071 to 1073 may berealized by a mechanical configuration or may be realized by anelectrical configuration.

Modification Example 2 When Threshold Value in Demodulation Unit isFixed

In the embodiment, although the threshold value for binarization isdynamically changed, in order to further improve precision of bitdetermination, the demodulation unit 22 in the reception apparatus 2 mayhave a configuration shown in FIG. 14, and a fixed threshold value maybe used in combination. A demodulation unit 22 a according toModification Example 2 is different from the demodulation unit 22 shownin FIG. 6 in that binarization units 2261-1 to 2263-1, to which theoutputs from the LPFs 2241 to 2243 are input directly without passingthrough the DC cut units 2251 to 2253, are provided. In thismodification example, the subtracters 2231 to 2233, the DC cut units2251 to 2253, the binarization unit 2261, the binarization units 2261-1to 2263-1, and the data detection unit 230 function as frame generationunit. Each configuration of the demodulation unit 22 a may be realizedby hardware or may be realized by cooperation of hardware and software.

To the data detection unit 230, bit data (CH1 bit data d, CH2 bit datad, and CH3 bit data d) shown in FIG. 6 passing through the DC cut units2251 to 2253 is input, and bit data (CH1 bit data z, CH2 bit data z, andCH3 bit data z) without passing through the DC cut units 2251 to 2253 isinput. The data detection unit 230 performs binarization for CH1 bitdata d, CH2 bit data d, and CH3 bit data d while dynamically changingthe threshold value th as in the embodiment, and performs binarizationfor CH1 bit data z, CH2 bit data z, and CH3 bit data z using a fixedthreshold value (in this case, 0). The data detection unit 230 generatesa frame using blocks, which show satisfactory results (results in whichno error occurs during demodulation), among blocks demodulated using thetwo kinds of threshold values.

Modification Example 3 Omission of Various Filters

Although the configuration of the demodulation unit 22 or 22 a shown inFIG. 6 or 14 uses the HPF 221, when a signal belonging to a band otherthan the modulation band is not included in the modulated audio signal Aso much, or when the influence of the signal is negligible, filtering bythe HPF is not be necessarily performed. Similarly, the LPF in themodulation unit 10 or the demodulation unit 22 is not required when theinfluence of the absence of the LPF is negligible.

Modification Example 4 Number of Blocks and Number of Frequency Bands

In the embodiment, although the number of blocks forming a frame isthree, the present invention is not necessarily limited thereto.Although the number of frequency bands F1, F2, and F3 for use inmodulation is three, the present invention is not necessarily limitedthereto.

When the number of blocks is greater than the number of frequency bands,the substantial transmission rate of transmission data decreases. On theother hand, when the number of frequency bands is greater than thenumber of blocks, the frequency bands are too many and a redundantconfiguration is made. Insofar as the decrease in the transmission rateor the redundant configuration is permitted, the number of blocks may bedifferent from the number of frequency bands. For example, the number ofblocks forming one frame may be six, and the number of frequency bandsfor use in modulation may be three. A way of deciding the number ofblocks forming one frame and the number of frequency bands for use inmodulation is arbitrary.

When the number of blocks forming a frame and the number of frequencybands for use in modulation are n in common such that the number ofblocks forming a frame is n (where n is a positive integer, and the sameapplies to the following description) and the number of frequency bandsF1, F2, and F3 for use in modulation is n, the demodulation unit 22separates an audio signal output from the microphone 20 into signalcomponents belonging to the n frequency bands, and demodulates a blockbased on each signal component for each time period of 1/n of a timeperiod necessary for collecting sound, on which one frame issuperimposed and which belongs to one of the n frequency bands. In thiscase, in connecting blocks selected from a demodulated block group togenerate a frame, the demodulation unit 22 selects the blocks such thata time period necessary for collecting sound, on which the selectedblocks are superimposed, is close to a time period of 1/n of a timeperiod necessary for collecting sound, on which one frame issuperimposed. In this way, when the number of blocks forming a frame andthe number of frequency bands for use in modulation are the same, it ispossible to efficiently use the frequency bands.

In the embodiment, the transmission apparatus 1 repeatedly performstransmission in units of frames without dividing into units of blocks,and the reception apparatus 2 cuts the received modulated audio signalin units of blocks and connects the blocks to generate a frame, it isnot necessarily so, and the transmission apparatus 1 may transmit datacorresponding to a frame while dividing into units of blocks, and thereception apparatus 2 may connect the blocks to generate a frame. Inthis case, since a header or the like can be attached to each block tobe transmitted, and the identifier of each block can be described in theheader, the reception apparatus 2 easily identifies each block referringto the identifier.

Modification Example 5 Procedure of Data Detection

In the embodiment, although the prescribed block selection methodincluding the four kinds of procedures of the procedure 1 to theprocedure 4 is assumed, if the condition that blocks are selected suchthat the time period necessary for collecting sound, on which theselected blocks are superimposed, becomes shorter is satisfied, a blockselection method other than the above-mentioned four kinds isconsidered. For example, in an environment in which multipath fadingoccurs and noise is likely to be included in a signal in each frequencyband for use in modulation, when transmitting one frame, a time periodlonger than a time period necessary for transmitting one frame may betaken. The reason for the use of the block selection method includingthe four kinds of procedures is that transmission quality at the sametime or in the same frequency band as a certain block a (correspondingto the block a1 of FIG. 12), in which the synchronization symbol issuccessfully extracted, is considered to be high, and if block selectionmethods are too many, the calculation load of the data detection unit230 increases or erroneous detection increases.

Modification Example 6 Frequency Bands in Pairs

In the embodiment, although near frequencies are paired, such as thefrequency bands f1=18000 Hz, f1′=18400 Hz, f2=18800 Hz, f2′=19200 Hz,f3=19600 Hz, and f3′=20000 Hz, for example, distant frequencies may bepaired, such as the frequency bands f1=18000 Hz, f2=18400 Hz, f3=18800Hz, f1′=19200 Hz, f2′=19600 Hz, and f3′=20000 Hz. For example, when aphenomenon, such as multipath fading or noise mixing, occurs in acertain frequency band, a frequency band comparatively near thefrequency is also influenced. Accordingly, as in this modificationexample, if distant frequencies are paired, improvement in tolerance forthe above-described phenomenon can be expected.

In the embodiment, the two narrow-band frequency bands are used, forexample, when transmitting the bit “1”, a signal belonging to thefrequency band f1 is output at a predetermined value and a signalbelonging to the frequency band f1′ is not output, and when transmittingthe bit “0”, a signal belonging to the frequency band f1 is not outputand a signal belonging to the frequency band f1′ is output at apredetermined value. In order to realize a transmission rate twice, fournarrow-band frequencies (frequency bands f1, f1′, f01, and f01′)belonging to the frequency band F1 are prepared. For example, whentransmitting the bits “1, 0”, a signal belonging to the frequency bandf1 is output at a predetermined value and a signal belonging to thefrequency band f1′ is not output. A signal belonging to the frequencyband f01 is not output and a signal belonging to the frequency band f01′is output at a predetermined value. For example, when transmitting thebits “0, 1”, a signal belonging to the frequency band f1 is not outputand a signal belonging to the frequency band f1′ is output at apredetermined value. A signal belonging to the frequency band f01 isoutput at a predetermined value and a signal belonging to the frequencyband f01′ is not output. In these cases, however, when focusing on onlythe frequency bands f1 and f1′, the outputs of the signals belonging tothe two narrow-band frequencies corresponding to the frame are invertedaccording to the value of each bit forming the frame, thereby generatinga modulated signal.

In the embodiment, although the frequency band of the carrier wave to bemodulated is a frequency band higher than a human audible frequencyband, the present invention is not necessarily limited thereto.

Modification Example 7 Propagation Medium of Sound

In the above-described embodiment, although it is assumed that the airis a medium through which sound propagates, in addition to gas otherthan the air, for example, a solid, such as a building, a structure, orfurniture, or a liquid, such as water, may be used. When a mediumthrough which sound propagates is a solid, the transmission apparatus 1includes a vibration unit for generating vibration according to a signaloutput from the output unit 11, instead of the speaker 12, and thereception apparatus 2 includes a vibration detection unit, such as anacceleration sensor, for detecting vibration of the solid, instead ofthe microphone 20. When sound is emitted from the solid which vibratesby the vibration unit of the transmission apparatus 1, the receptionapparatus 2 may include the microphone 20 as in the embodiment.

Modification Example 8 Transmission Start Timing

The term “transmission start timing” according to the present inventionincludes substantially the timing which is regarded as the timing atwhich frame transmission starts, such as the timing at which processingfor supplying audio data to the modulation unit 10 for sound emissionstarts, or the timing at which processing for superimposing transmissiondata on audio data in the modulation unit 10 starts, in addition to thetiming at which an audio signal on which transmission data issuperimposed is supplied from the output unit 11 to the speaker 12 andsound emission starts.

Modification Example 9 Threshold Value for Use in Bit Determination

The threshold value when bit determination is performed based on thedifferential signal may be a fixed threshold value, not the thresholdvalue th which varies over time as in the embodiment.

Modification Example 10 Program

The present invention can be specified as a program for causing acomputer to realize the same function as the transmission apparatus 1 orthe reception apparatus 2 or a recording medium, such as an opticaldisc, having the program stored therein. The program according to thepresent invention may be provided in the form of being downloaded to thecomputer through a network, such as Internet, installed, and madeavailable.

This application is based on Japanese Patent Application No.2013-032506, filed on Feb. 21, 2013, and Japanese Patent Application No.2013-246685, filed on Nov. 28, 2013, the contents of which areincorporated herein by reference.

INDUSTRIAL APPLICABILITY

The present invention is useful in that it is possible to increasepossibility of extracting data from sound in a system which transmitsdata using sound as a transmission medium.

REFERENCE SIGNS LIST

1: transmission apparatus, 2: reception apparatus, 10, 10 a: modulationunit, 11: output unit, 12: speaker, 20: microphone, 21: input unit, 22,22 a, 22 b, 22 c, 22 d: demodulation unit, 101, 1021 to 1023, 2241 to2243: LPF, 1031 to 1033: VCO, 1041, 1042: delay device, 105, 2231 to2233: adder, 220: bit decoding unit, 221: HPF, 222: STFT unit, 2251 to2253: DC cut unit, 2261 to 2263, 2261-1 to 2263-1: binarization unit,230: data detection unit, 240: data detection trigger generation unit,2411 to 2413: FFT unit, 2421 to 2423: normalization unit, 243:multiplier, 244: signal level calculation unit.

The invention claimed is:
 1. A demodulation apparatus comprising: aseparation unit configured to use a frame corresponding to one unit oftransmission data and having a transmission start timing delayed by apredetermined time period to separate an audio signal of sound emittedaccording to a modulated signal generated by modulating a carrier wavein a frequency band that is different depending on the transmissionstart timing into signal components belonging to the respectivefrequency bands; and a frame generation unit configured to demodulate ablock forming a part of the frame based on a part of the signalcomponent separated by the separation unit, select and demodulate ablock forming a remaining part of the frame according to a prescribedselection-procedure, and connect the demodulated blocks to generate theframe, wherein the prescribed selection procedure includes a pluralityof different procedures relating to a block selection, and wherein in acase where an error in demodulating an entirety of the frame in one ofthe plurality of procedures occurs, the frame generation unit performs asubsequent procedure among the plurality of different procedures.
 2. Thedemodulation apparatus according to claim 1, wherein: frequency bands ofcarrier waves modulated by modulated signal generation unit are nfrequency bands, where n indicates a positive integer, and the framegeneration unit demodulates the block based on each signal component foreach time period of 1/n of a time period in which one frame issuperimposed in any of the n frequency bands.
 3. The demodulationapparatus according to claim 1, wherein: each of the frequency bandsincludes a plurality of narrow-band frequencies having a bandwidthnarrower than the corresponding frequency band, the modulated signalgeneration unit generates the modulated signal by causing outputs ofsignals belonging to two narrow-band frequencies corresponding to theframe to be inverted between the signals according to a value of eachbit of the frame, the separation unit separates the audio signal intosignals belonging to the two narrow-band frequencies included in each ofthe frequency bands, and the frame generation unit compares a differencebetween the signals belonging to the two narrow-band frequencies with athreshold value and decodes each value of the bits to demodulate theblock.
 4. The demodulation apparatus according to claim 3, wherein theframe generation unit calculates an upper envelope and a lower envelopeof each signal component separated by the separation unit and uses atime-varying value between the calculated upper envelope and the lowerenvelope as the threshold value.
 5. An audio communication systemcomprising: a transmission apparatus for emitting an audio signal, onwhich transmission data to be transmitted is superimposed, as sound; anda reception apparatus for extracting the transmission data from thesound emitted from the transmission apparatus, wherein the transmissionapparatus includes: a delay unit configured to delay a transmissionstart timing of a frame corresponding to one unit of the transmissiondata by a predetermined time period; a modulated signal generation unitconfigured to generate a modulated signal by modulating a carrier wavein a frequency band that is different depending on the transmissionstart timing using the frame whose transmission start timing is delayedby the delay unit; and a sound emission unit configured to emit thesound according to the modulated signal generated by the modulatedsignal generation unit, and wherein the reception apparatus includes: asound collection unit configured to collect the sound emitted from thesound emission unit and output an audio signal; a separation unitconfigured to separate the audio signal output from the sound collectionunit into signal components belonging to the respective frequency bands;and a frame generation unit configured to demodulate a block forming apart of the frame based on a part of the signal component separated bythe separation unit, select and demodulate a block forming a remainingpart of the frame according to a prescribed selection procedure, andconnect the demodulated blocks to generate the frame, wherein theprescribed selection procedure includes a plurality of differentprocedures relating to a block selection, and wherein in a case where anerror in demodulating an entirety of the frame in one of the pluralityof procedures occurs, the frame generation unit performs a subsequentprocedure among the plurality of different procedures.
 6. Anon-transitory computer-readable storage medium storing a programexecutable by a computer to execute a method of demodulating for ademodulating apparatus, the method comprising: a separation step ofusing a frame corresponding to one unit of transmission data and havinga transmission start timing delayed by a predetermined time period toseparate an audio signal of sound emitted according to a modulatedsignal generated by modulating a carrier wave in a frequency band thatis different depending on the transmission start timing into signalcomponents belonging to the respective frequency bands; and a framegeneration step of demodulating a block forming a part of the framebased on a part of the signal component separated by the separationstep, selecting and demodulating a block forming a remaining part of theframe according to a prescribed selection procedure, and connecting thedemodulated blocks to generate the frame, wherein the prescribedselection procedure includes a plurality of different proceduresrelating to a block selection, and wherein in a case where an error indemodulating an entirety of the frame in one of the plurality ofprocedures occurs, the frame generation step performs a subsequentprocedure among the plurality of different procedures.
 7. A method ofdemodulating in a demodulation apparatus, the method comprising: aseparation step of using a frame corresponding to one unit oftransmission data and having a transmission start timing delayed by apredetermined time period to separate an audio signal of sound emittedaccording to a modulated signal generated by modulating a carrier wavein a frequency band which is different depending on the transmissionstart timing into signal components belonging to the respectivefrequency bands; and a frame generation step of demodulating a blockforming a part of the frame based on a part of the signal componentseparated by the separation step, selecting and demodulating a blockforming a remaining part of the frame according to a prescribedselection procedure, and connecting the demodulated blocks to generatethe frame, wherein the prescribed selection procedure includes aplurality of different procedures relating to a block selection, andwherein in a case where an error in demodulating an entirety of theframe in one of the plurality of procedures occurs, the frame generationstep performs a subsequent procedure among the plurality of differentprocedures.
 8. A demodulation apparatus comprising: a separation unitconfigured to use a frame corresponding to one unit of transmission dataand having a transmission start timing delayed by a predetermined timeperiod to separate an audio signal of sound emitted according to amodulated signal generated by modulating a carrier wave in a frequencyband that is different depending on the transmission start timing intosignal components belonging to the respective frequency bands; and aframe generation unit configured to demodulate a block forming a part ofthe frame based on a part of the signal component separated by theseparation unit, select and demodulate a block forming a remaining partof the frame according to a prescribed selection procedure, and connectthe demodulated blocks to generate the frame, wherein the prescribedselection procedure includes a plurality of different proceduresrelating to a block selection, and wherein, in a case where the framegeneration unit completes demodulating the block forming the remainingpart of the frame and correctly demodulates an entirety of the frame inone of the plurality of different procedures, the frame generation unitdoes not perform any subsequent procedure among the plurality ofdifferent procedures.
 9. The demodulation apparatus according to claim1, wherein one of the plurality of different procedures selects eachblock forming the remaining part of the frame from another signalcomponent that is different from the signal component from which thepart of the frame is demodulated.
 10. The demodulation apparatusaccording to claim 1, wherein one of the plurality of differentprocedures selects each block forming the remaining part of the framefrom a signal component same as the signal component from which the partof the frame is demodulated.
 11. The demodulation apparatus according toclaim 1, wherein at least one of the plurality of different proceduresis different from others in a required time for the block selection.